1. Field of the Invention
The present invention relates to a method of and an apparatus for generating high-resolution binary image data with respect to linear edges. More particularly, it relates to a method of and an apparatus for scanning binary (e.g., black-and-white) originals including characters, maps, illustrations or the like in an image scanning/recording apparatus such as an electronic process scanner or a facsimile to input image data of multi-gradation (the grandient corresponds to black areas in read pixels in the black-and-white case, for example), thereby to generate binary image data having a higher resolution than the read resolution from the inputted multi-gradation image data.
2. Description of the Prior Art
In the case of scanning originals to input image data into a general image scanning/recording apparatus, the resolution of the obtained input image data is restricted by technical or economical limitations. In the case of an electronic process drum scanner, for example, a practical number of scanning lines is about 400 lines/inch (input resolution of 62.5 .mu.m square).
On the other hand, higher resolution is required in the field of commercial printing and publication printing requiring high quality, and it is believed that the resolution preferably exceeds 1500 lines/inch (16.9 .mu.m square) so that no indentation appears on edge portions of the output images in a binary image processing system for scanning/inputting binary images and performing some image processing to output the same in the same size in magnification, for example. On the premise of the same-size processing, therefore, high resolution of the said degree is also required in the input side. However, a binary image input system capable of reading at a resolution exceeding 1500 lines/inch is considerably high-priced. Further, the process time is increased when sub-scanning is made per line at such a resolution.
Therefore, the inventor has proposed a high resolution technique of obtaining binary image data higher in resolution than input image data and with a higher fidelity than the original through a conventional low-resolution image input system without employing a high-resolution binary image input system such as in copending application U.S. Ser. No. 915,680 entitled "Method of and Apparatus for Generating High-Resolution Bilevel Image Data". In this method, however, reproducibility with respect to the original is extremely high, whereby fine fluctuation included in linear edges of the original is also reproduced in fidelity. Accordingly, the corresponding highly resolved edge portions are indented. Indentations particularly caused in linear edges parallel with or perpendicular to the scanning lines (i.e., in conformity to the main scanning or sub-scanning direction) are unsightly and the data are increased in number when the same are subjected to run-length compression in past processing to disadvantageously increase the process time.